The present invention relates to a process for removal of impurities from impure sulphide melts obtained in pyrometallurgical processing of sulphidic complex and mixed ores or concentrates. By means of the process harmful impurities such as arsenic, antimony, bismuth and lead are removed from sulphide melts containing copper, nickel, cobalt and iron as the main metals.
With resources of pure, easily processed ores becoming exhausted the processing of the afore-mentioned complex and mixed ores has become a necessity. Nevertheless, in processing these raw materials by current methods, so-called harmful impurities follow the main metal through the various process stages to such an extent that the contents of these impurities in the end product exceed permitted limits. Typical impurity contents, for example in raw copper from which conductor copper is produced electrolytically, are: Pb 2000, As 1000, Sb 300, Bi 100 g/t.
Conventional processes for the production of copper (reverberatory furnace and conversion) do not yield sufficiently low impurity contents in respect of arsenic, antimony and bismuth when ores and concentrates containing large amounts of these impurities have to be used. The maximum arsenic content of concentrate is of the order 500-2500 g/t and during the various process stages 50-90% of the impurities can be removed in the slag or volatilized.
In the following some new pyrometallurgical processes are examined which have been developed for the removal of impurities from molten sulphide matte.
The sulphide matte produced when sulphidic concentrates are smelted is usually processed in a converter. In the converter process the elimination of impurities has been improved by means of plant technology, by adjusting the process variables, by changing the composition of the slag and by feeding additives into the converter.
Raising the temperature of conversion improves conditions for the volatilization of impurities. The temperature can be increased by such means as starting the conversion of the copper matte from a "lean" matte, i.e. a matte in which the copper content is low. At the same time the duration of the slag-blowing stage is lengthened and the elimination of volatile components (As, Sb etc.) is improved. The drawback of this method is the large quantities of slag which are produced. The temperature can be raised also e.g. by the addition of coke, which at the same time promotes the volatilization of impurities bound in the slag (e.g. Zn and Pb).
By adjusting the composition of the slag the elimination of secondary components can be increased and selective slagging-off of impurities is possible. Particularly additions of alkaline earthsand alkali oxides to the slag have improved slagging of impurities.
Certain impurities which have not been slagged off or volatilized during slag blowing concentrate in the metallic phase produced during the initial stage of blister blowing. By removing this so-called "first drop" from the converter before continuing blister blowing the impurity content of the principal metal being produced is decreased.
Vacuum treatment is a suitable refining method for both sulphide matte which contains iron and for the so-called converter matte which is obtained after slag blowing. In experiments made on the vacuum processing of converter matte according to the publication Kametani, H., Yamauchi, C., Murao, K., Hayashida, M., Trans. JIM 14, (1973), p. 218-223, elimination of zinc and lead was good and the elimination percentages for antimony and bismuth were in the range 32-76%. Arsenic was not eliminated by vacuum processing from converter matte, but for sulphide matte containing iron the elimination percentages were in the range 64-93%.
Impurity components can be effectively removed from molten sulphide matte by selective chlorination. According to U.S. Pat. No. 3,802,870 nickel matte can be refined by extracting impurities, principally Cu, Co, Cd, Fe, Pb, Mg, Sn and Zn, into a molten salt mixture containing chlorides of metals belonging to group IA or IIA of the periodic system. Nickel chloride or gaseous chlorine is used as the chlorinating agent. The process temperature is in the range 750.degree.-900.degree. C. Because the principal components are Cu, Co and Fe an effort is made to keep the sulphur content of the matte below the stoichiometric sulphur amount as calculated from the sulphur requirement of Ni.sub.3 S.sub.2. The sulphur content of the matte should be in the range 18-26%.
According to U.S. Pat. No. 3,938,989 arsenic, antimony, bismuth, selenium and tellurium can also be removed from nickel matte by chlorination. These impurities chlorinate as gaseous chlorides which volatilize from the melt. To reduce volatilization of nickel chloride the molten matte is covered by a layer of molten halide. A prerequisite for the elimination by chlorination particularly of arsenic is a high sulphur content in the molten nickel matte. In the patent a minimum sulphur content of 28% is mentioned, preferably the sulphur content is in the range 30-32.5%. The sulphur content of the melt is adjusted using elemental sulphur or hydrogen sulphide before chlorination. The temperature of the chlorination treatment is 750.degree.-1000.degree. C.
Chlorination has been applied in accordance with Finnish Pat. No. 55 357 also to the refining of metal sulphide melts in which there exists a solubility gap between the metal being refined and its sulphide. An example of such a melt is that of copper sulphide. As, Sb, Bi and Pb arise as possible impurities to be removed. The impurities are removed as volatile chlorides by selective chlorination using either gaseous chlorine or a mixture of chlorine and nitrogen in the temperature range 1150.degree.-1250.degree. C. Before chlorination the sulphide melt is saturated with sulphur to move the composition of the melt away from the melt solubility gap. The sulphidation can be carried out using elemental sulphur vapour, hydrogen sulphide, covellite or pyrites. When large amounts of impurities are present the melt must be sulphidated at intervals during the chlorination treatment or chlorination must be carried out using a gaseous mixture containing elemental sulphur so that the composition of the melt remains outside the solubility gap. In this way the activities of copper and its sulphide fall so much that selective chlorination of impurities from the melt is possible. No differences were observed between the impurity contents of unsulphurated and sulphurated converter matte, the impurity contents only being reduced upon chlorination of the sulphurated matte and the impurities consequently escaping as chlorides. In withdrawn Finnish (Pat. Application No. 783186) relating to the chlorination of copper matte and copper nickel matte, Pb, As, Bi, Zn, Sb and Cd are removed by chlorination from the molten matte as volatile chlorides. Copper chloride, nickel chloride or gaseous chlorine is used as the chlorinating agent. To promote the removal of impurities inert gas is blown into the melt after chlorination and possibly during chlorination. Together with chlorine, air can also be blown into the melt. The chlorination treatment is carried out in the temperature range 800.degree.-1200.degree. C. During the whole time of the treatment the layer of molten matte can be covered by a salt melt. Before chlorination the sulphur content of the melt is adjusted so that the sulphur content corresponds to at least 1.05 times the calculated stoichiometric sulphur requirement of the copper, iron, nickel and cobalt. The sulphur content is increased using solid elemental sulphur, sulphur vapour or gas containing sulphur.
Known processes for the removal of impurities are either expensive, because they require separate treatment of fly dust, vacuum treatment of the matte, alkali slagging etc., or else they give rise to occupational health or materials problems, for example as the result of chlorination. Moreover, for example in chlorination treatment considerable quantities of gases containing SO.sub.2 and Cl.sub.2 are produced and the treatment of these gases by a sulphuric acid plant is difficult.
It is an object of the present invention to provide a process by which a sulphide melt containing large amounts of arsenic and other so-called troublesome impurities can be refined without the aforesaid problems by sulphurating the molten sulphide matte and treating it with inert gas either in the ladle or in the converter. The advantage of this process is low capital cost for plant and low additional running costs, and also that the gases are suitable for the manufacture of sulphuric acid.